As a method for holding a frame for a shadow mask within a face panel in a color cathode ray tube, a three-pin method has conventionally been used in which the frame is held through three springs located on the two short sides and one long side respectively. In this method, a pin is located in the center on the long side and two pins are located on the short sides at a predetermined distance from the center toward the long side that does not have a pin (see Publication of Japanese Patent Application (Tokkai Sho) No. 60-232639).
A conventional color cathode ray tube will be described below in which a frame for a shadow mask is held within a face panel using the three-pin method. FIG. 7 shows a sectional front view of a color cathode ray tube in the prior art. As shown in FIG. 7, a shadow mask 52 attached to a frame 51 is located on the inside of a face panel 50. The frame 51 for the shadow mask is held by the face panel 50 through a spring 56 supported by a pin 55 provided on the inner surface of the face panel 50. One end of the spring 56 is fixed to the frame 51 at two welding points 53 and 54, and the other end is supported by the pin 55. The distance (operation length) 57 from the pin 55 to the welding point 53 of the spring 56 that is closer to the pin 55 is set at the same value among the three springs 56a, 56b and 56c.
Due to the increase of electric current associated with the high luminance of recent color cathode ray tubes, a phenomenon is caused in which the entire shadow mask thermally expands to form a dome shape in the direction of the phosphor surface (hereinafter such a phenomenon is referred to as "the entire doming"). In order to solve the entire doming problem, an invar material having a lower coefficient of thermal expansion than iron has been used as the material of the shadow mask.
Also, in order to obtain higher definition images in computer display monitors, the phosphor surface of the face panel is formed in dot shape and precisely positioned so that electron beams passed through the dot-shaped holes of the shadow mask accurately strike phosphor dots on the phosphor surface.
However, the conventional structure as described above cannot fully meet the requirements for recent display monitors. Since the reverse mode in which a white background is displayed on the screen is common as a use state of a recent display monitor, and a high luminance of 30 Ft-L as the brightness of displayed images and full scan as the display size is common, the amount of electric current increases. Thus, the entire doming phenomenon caused by the thermal expansion of the shadow mask and the misalignment of the dots of the shadow mask and the phosphor dots on the phosphor surface cannot be fully prevented. Therefore, due to the misalignment of the position where electron beams strike and the phosphor dots, the efficiency of emitting light decreases, thus deteriorating the white quality of the screen, for example, causing a decrease in luminance output and a change in chromaticity.